1. Field of Application
The present invention relates to a directivity control antenna apparatus for use in base stations and mobile stations of a mobile communication system such as a radio paging system, cellular telephone system, etc.
2. Prior Art
In the prior art, methods such as time division multiplexing (TDMA), frequency division multiplexing (FDMA), code division multiplexing (CDMA), etc., have been used in the field of mobile communication, as methods for communicating with a plurality of mobile stations within areas which have been allocated to respective base stations of a mobile communication system. With time division multiplexing, a channel having a single frequency is divided into equidistant intervals along the time axis to provide communication slots, for enabling communication between a plurality of mobile stations and a base station, while with frequency division multiplexing this is accumplished by using a plurality of different frequency channels, Alternatively, with code division multiplexing, a plurality of mobile stations execute spectrum dispersion modulation by using data encoded such as to ensure there is no mutual correlation between the respective received signals which are obtained by a base station from the various mobile stations, to thereby enable communication between the mobile stations and the base station within the same frequency band.
With such a prior art system, in general communication is only possible between a base station and those mobile stations which are located within the service area of the base station, i.e. an area of nominally predetermined size. In practice, the level of signal strength received by a base station from a mobile station is usually employed as an indication of whether the mobile station is within the service area of that base station. Also, in general, the radio transmission frequencies which are used by a base station and by the mobile stations which are located within the service area of that base station are predetermined such that these will not result in interference with communication between any adjacent base station and those mobile stations which are within the service area of the adjacent base station.
When such prior art technology is used, the number of mobile stations which can be accommodated by the service area of a single base station is limited to the total number of communication slots that are allocated to the mobile stations, in the case of time division multiplexing, while that number of mobile stations is limited to the total number of frequency channels, in the case of frequency division multiplexing, and is limited by the degree of effectiveness of interference prevention (which is determined by the ratio of the data transfer rate to the spectrum dispersion chip rate) in the case of code division multiplexing. There are limitations on frequency resources, so that there are limitations on the number of channel frequencies and frequency bandwidth which can be allocated to mobile communication systems, and this places an upper limit on the number of mobile stations which can be accommodated by a mobile communication system. Moreover since the service area of a base station and the frequencies or number of TDMA slots which can be used by a base station for communication with mobile stations are fixed, cases can occur in which one of the base stations has communication channels available, while the slots or communication channels of an adjacent base station are completely filled. In the prior art, in such a case, i.e. in which there is communication channel capacity available in the overall system, it may not be possible to achieve communication in some instances due to lack of communication channel capacity in one or more base stations.